14 Fine tuning

The Standard Model has about twenty-six freely adjustable parameters. Certain combinations of these parameters appear to be remarkably fine-tuned for life.[1–3] As Stephen Hawking wrote,[4]

The laws of science, as we know them at present, contain many fundamental numbers…. The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life.

Example 1: Supernova explosions only occur if a certain numerical relation involving the dimensionless coupling constants of the weak and gravitational interactions and the proton-electron mass ratio are approximately satisfied. If the weak coupling constant were too large, the neutrinos released by the collapse of the core of a star could not reach the stellar envelope before losing most of their energy. And if it were too small, the neutrinos would escape with most of their energy. In either case, the star would fail to explode.

Example 2: Stars rely on two mechanisms for transporting energy from their cores to their surfaces: radiation and convection. Astronomical observations indicate that only stars which are at least partially convective have planets. On the other hand, only radiative stars explode. So stars of both types are needed, and this calls for a delicate balance between the dimensionless coupling constants of electromagnetism and gravity and the proton-electron mass ratio. If gravity were slightly stronger, only radiative stars would exist, and if it were slightly weaker, only convective stars would exist.

Example 3: The synthesis of carbon is a two-step process. The first step is the formation of an 8Be nucleus out of two 4He nuclei (alpha particles), the second the formation of a 12C nucleus out of the 8Be nucleus and another 4He nucleus. The probability of this process would be extremely small, were it not for two “coincidences”: the 8Be ground state has almost exactly the energy of two alpha particles, and 8Be+4He has almost exactly the energy of an excited state of 12C. In other words, the 8Be ground state “resonates” with a system comprising two alpha particles, and the excited 12C state resonates with a systems comprising 8Be and 4He. The existence of the second resonance was predicted by Fred Hoyle before its actual observation, based on the observed abundance of carbon in the Universe and the necessity for it to be formed in stars. The energy at which this resonance occurs depends sensitively on the interplay between the strong and the weak nuclear interactions. If the strong force were slightly stronger or slightly weaker — by just 1% in either direction — then the binding energies of the nuclei would be different, and the requisite resonance would not exist. In that case, there would be no carbon or any heavier elements anywhere in the Universe. “I do not believe,” Hoyle concluded,[5] “that any scientist who examined the evidence would fail to draw the inference that the laws of nuclear physics have been deliberately designed with regard to the consequences they produce inside the stars.”

It is self-evident that the actual features of the Universe impose constraints on the laws by which it is governed.

If the universe contains (carbon-based) life, and if it is governed by general relativity and the Standard Model, then the adjustable parameters of these theories must be so constrained as to allow for the evolution of life. This truism is known as the “weak anthropic principle.” What is nevertheless remarkable is the number of constraints that have been uncovered and, in consequence, the extent to which those parameters are fine-tuned for life.

But there is no need to invoke life. It suffices to have objects that have spatial extent, that are composed of a finite number of objects without spatial extent, and that neither explode nor collapse as soon as they are created. If such objects are to exists, we must have quantum mechanics, special relativity, general relativity, and the Standard Model — the latter two at least as effective theories — and the adjustable parameters of these theories must be so constrained as to allow for the existence of such objects. Quantum mechanics presupposes macroscopic objects, including objects that can function as outcome-indicating devices, and it seems all but certain that the existence of such devices calls for elements whose existence depends on stellar nucleosynthesis and supernova explosions. If so, it calls for some of the same fine tuning that has been shown to be necessary for life.